16.4.1 Welding

Jonghyan et al. [6] reported the melting characteristics of various metals following irradiation by combined lasers with different wavelengths. A Q-switched YAG laser and pulsed YAG laser were combined both through different optics lines and the same optics line as shown in Figs 16.8 (a) and (b), respectively. Figure 16.9 shows cross sections of a weld bead obtained both by single-beam irradiation and twin-beam irradiation. In this example, the Q-switch and pulse YAG laser were applied through different optical lines. The size and depth of the molten pool were increased after twin-beam irradiation. Figure 16.10 shows a cross section of weld bead obtained using twin-beam irradiation in different metals. Keyhole type penetration was observed and the molten area and depth of the molten pool increased following twin-beam laser irradiation (Fig. 16.10). The effect of twin-beam laser irradiation is schematically shown in Fig. 16.11. In cases where a highly reflective and thermally conductive material was used, two layers of molten pool were observed. Contrastingly, in cases where materials with a low thermal conductivity were used, deeper penetration was obtained. Jonghyan et al. [6] reported that the absorptivity of the laser increased when twin-beam laser irradiation was used by 21%, 29% and 25% for a pure copper, A5083 and SUS304, respectively.

Shibata et al. [7] studied the effect of twin spot beam configuration on the overlap joint welding of aluminum alloys. Two different Nd:YAG laser beams were separately applied as shown in Fig. 16.6. During this study the distance between the two beams was changed. Figure 16.7 shows the beam profile obtained at different beam distances. Tandem beam and parallel beam configurations were applied as shown in Fig. 16.12. Figure 16.13 shows a cross section of a weld bead. The penetration depth decreased with the increase in beam distance for both beam configurations. When the beam distance became too short, however, the welding process became unstable. The configuration of twin beams should be selected depending on material thickness, laser power and scanning speed in order to obtain higher quality welding.

The effect of the twin-beam laser irradiation on porosity formation was studied by Shibata et al. [5], who changed the welding configuration and applied the beam to an aluminum alloy. Two different Nd:YAG lasers were applied with different optical lines. Figure 16.14 shows the relationship between the level of porosity and the beam distance in both the tandem and the parallel beam configurations. The figure shows that the level of porosity decreased as the beam distance increased, regardless of the beam configuration. A large opening distance and a shallow keyhole depth might, therefore, suppress the occurrence of porosity. Twin laser beam irradiation proved to be an effective method of controlling keyhole shape. Hayashi et al. [8] also studied the effect of twin-beam irradiation on porosity formation using stainless steel. A CO₂ laser beam was split into two beams using a mirror to obtain the tandem beam configuration. According to the X-ray transmission in-situ observation during the welding process, two types of keyhole were observed as shown in Fig. 16.15. Where the two beams were separately applied, as shown in Fig. 16.15(a), porosity formed at the bottom of the keyhole. Where one large keyhole formed, as shown in Fig. 16.15(b), however, the formation of porosity was suppressed due to the change in the flow behavior.

Another expected advantage of applying twin beams is their effect on gap tolerance as reported by Ohashi et al. [9]. In this study, two YAG lasers were used and applied through different optical lines and both tandem and parallel laser beam configurations were applied. In the case without a filler wire, the parallel beam configuration had a greater tolerance than the tandem beam configuration, as shown in Fig. 16.16. Figure 16.17 shows cross sections obtained by applying the twin beam to a filler wire. Welding was available until the gap dropped below 1.6 mm for both the tandem beam and the parallel beam configurations. Where the gap was much larger, however, the energy of the laser beam was spent in heating the wire and the heat input into the base material therefore decreased. One solution to the problem is shown in Fig. 16.18. In cases using the parallel beam configuration, the irradiation area can be controlled by changing the beam distance. By using this technique, heat input at the bottom of the gap is increased and gap tolerance is effectively improved.

Iqbal et al. [10], [11] applied the twin-beam technique for the welding of zinc coated steel. An Nd:YAG laser was used as an advancing beam and a CO₂ laser was used as the following one in the tandem twin-beam configuration. The front beam (Nd:YAG laser) cut a slot to remove the zinc coating and the back beam (CO₂ laser) welded a material as shown in Fig. 16.19. Figure 16.20 shows a schematic of the proposed welding method by the tandem twin-beam irradiation.

16.4.2 Dissimilar materials welding

The application of the twin-beam laser technique for dissimilar materials welding is described below [3]. In the majority of cases involving the welding of dissimilar materials, intermetallic phases are formed, which reduce the strength of the joint. In cases where the materials have very different melting temperatures, the formation of the intermetallic phase could be controlled using a solid-state–liquid-state reaction at the interface [12]. When there is only a minor difference in the melting temperature of the materials, however, such as in magnesium alloy/aluminum alloy dissimilar metals welding, the solid-state–liquid-state reaction cannot be achieved and the intermetallic phase can be easily formed in the molten pool. An alternative method for controlling the formation of the intermetallic phases will therefore be required.

AZ31B magnesium alloy and A5052-0 aluminum alloy plates of 1 mm in thickness were used in the present study. The Nd:YAG laser was used for welding with both single-beam and twin-beam with the parallel beam configuration that both beams were placed in perpendicular to the welding direction. Twin laser beams were obtained by splitting an original beam using a prism as shown in Fig. 16.1. In this study, the two wedge angles of the prism were 2° and 4°, which separated the beam distances into 0.87 and 1.60 mm, respectively. The welding configuration used is shown in Fig. 16.21. The laser was irradiated on the AZ31 plate, which was the upper plate of the lap joint configuration in the direct welding. The laser power used was 3 kW with CW.

Plate XVIII (between pages 328 and 329) shows cross sections of a weld pool for all welding configurations. The joint width and depth of the molten metal for the joints obtained are shown in Fig. 16.22. The weld bead width and depth of the molten metal slightly increased with a decrease in welding speed for single-beam welding and twin-beam welding with a 2° wedge prism. In the case of the twin beam with a 4° wedge prism, however, an increase in the depth of the molten metal was not significantly observed, but the weld width increased. Figure 16.23 shows the failure load and joining strength of the joints. It was found that an application of single-beam and twin-beam welding with a 2° wedge prism resulted in decreasing joining strength with a decrease in welding speed because deep penetration of the molten metal might cause the formation of thick intermetallic compounds. In the case of the twin-beam welding with a 4° wedge prism, however, the change in depth of the molten metal is very small resulting in an increase in the failure load and a negligible change in the joining strength even when the welding speed decreased.

The failure load of the joints could be improved by applying the twin beam to direct welding as just described. An intermetallic phase was still formed at the interface, however, which affected the strength of the joint. In order to control the composition of the material at the interface, a filler metal was applied. The filler wire used was an Al12Si eutectic alloy with a diameter of 1.2 mm. The wire was positioned at the edge of the upper plate. Both the single beam and twin beam were used. In the case of twin beam when welding with a filler metal, the tandem beam configuration with two beams placed parallel to the welding direction was used. A5052 was placed as an upper plate irradiated by the laser beam. The laser power used was 3 kW. For the twin-beam welding, a 4° wedge prism was used to split an original beam. The intensity ratio between the forward beam and the following beam was 1:2. A schematic illustration of the welding configuration for twin-beam laser welding with a filler wire is shown in Fig. 16.24.

According to observations made by a high-speed camera during the twin-beam laser welding process, it was confirmed that the forward beam was used only for pre-heating without melting the base metal. Plate XIX (between pages 328 and 329) shows cross-sectional observations of the joints welded with the filler wire for both single- and twin-beam welding. Full penetration always occurred during single-beam irradiation, whereas only partial penetration was observed during twin-beam irradiation. At the interface between the filler metal and the base metals, Mg₂Si and Al-Mg intermetallic phase was observed. Figure 16.25 shows the failure load of the joints welded with a filler wire. The failure load of the joints welded with the twin beam was higher than that of those welded with a single beam.

16.4.3 Surface modification

Laser cladding is a surface modification technique and has been used in applications such as preparing the valve sheet of an engine component in the motor industry. In the process, particles are mixed together with the molten metal at the surface. A composite or alloying material is then formed to improve the properties of the original material such as its ability to withstand corrosion, temperature and wear, etc. In this section, laser cladding using twin-beam irradiation on a magnesium alloy is discussed.

The material and particle used for the surface modification were an AZ31 magnesium alloy and Si. These can be expected to form Mg₂Si, which will increase the hardness and wear resistance of the alloy. An Nd:YAG laser was used and the beam was split by a prism with a wedge angle of 4°. Two configurations of twin beams were applied: (1) the parallel beam in which two beams were placed perpendicular to the laser scanning direction, and (2) the tandem beam in which two beams were placed parallel to the laser scanning direction. A Si powder layer with a thickness of 0.3 mm was placed on the surface of the base metal before irradiation of the laser beam. A schematic illustration of the laser cladding experiment is shown in Fig. 16.26.

The modified cladded surfaces following the single-beam and twin-beam irradiation are shown in Fig. 16.27. Two welding beads can clearly be observed in the case of the surface cladding using parallel beams. This separation of the beads becomes more apparent as the scanning speed increases. The bead width of the parallel beam surface was also small compared with that in the single beam case. Cross-sectional observations of the materials modified with single- and twin-beam irradiation are shown in Plate XX (between pages 328 and 329). In the case of the parallel twin-beam irradiation with lower laser scanning speed, Si powders were mixed well. The amount of non-melt Si powders increased with the laser scanning speed, however. The cross sections show that the depth of the molten pool created by the twin-beam irradiation was shallow compared with that created by single-beam irradiation. This was due to the low energy density of the laser beams in the twin-beam irradiation. If attention was paid to the one molten line, the Si compound was distributed along the molten pool, but the mixing was not well done inside the molten pool. In the middle section of the irradiation points of the two beams, granular-like Si compound was observed. It is speculated that the duration time for maintaining the molten pool decreases in the case of parallel twin-beam irradiation under the higher scanning speed conditions.

Observation of a weld bead created with the tandem-beam irradiation is shown in Fig. 16.27(c). The bead width was narrower with the parallel-beam irradiation when compared with the single-beam irradiation as previously mentioned. Bead width was also narrower in the case of the tandem-beam irradiation. The smooth surface of the material following irradiation with the tandem beams was obtained due to the long laser irradiation time. Plate XX (between pages 328 and 329) shows the cross section of the part subjected to the tandem laser beam cladding. The molten pool shape was completely different between the single-beam irradiation and the parallel twin-beam irradiation. In the cases of the single-beam irradiation and the parallel twin-beam irradiation, Si particles were found around the bottom of the molten pool and a lack of Si was also observed in the middle section. Contrastingly, in the case of the tandem-beam irradiation, Si particles were uniformly distributed in the molten pool. However, the increase in scanning speed meant that the molten metal and Si particles were not thoroughly mixed. Inside the molten pool, Mg₂Si was formed significantly. In the case of the tandem-beam irradiation, the following beam effectively mixed the material melted by the irradiation of the advancing beam and the total duration time of melting was much longer than that of either the single or parallel twin-beam irradiation. The tandem twin-beam irradiation is an effective method of inducing flow activity in the molten pool. Clusters of Si particles were not found around the bottom of the molten pool in laser clad material which had been irradiated using the tandem twin beam. Plate XXI (between pages 328 and 329) shows the cross section and Si distribution of the material after laser cladding using the tandem twin-beam irradiation. Around the interface between the base metal and the molten pool, the Si content continuously increased from the base metal to inside the molten pool. Plate XXII (between pages 328 and 329) shows the enlarged images of Plate XXI. A large number of Mg₂Si particles were formed. This was due to effective and thorough mixing of the molten metal induced by the tandem twin-beam irradiation.

16.4.4 Cutting

A diamond tool is conventionally used to cut brittle materials, since it provides a high cutting quality and productivity [13]. The unavoidable disadvantages of diamond tools, however, include the occurrence of pollution for the material and working environment, micro-cracking and a loss of the material cut due to finite thickness of the tool. Conventional laser cutting of metals usually causes the melting and vaporization of the material, which not only would not solve the conventional mechanical cutting process caused by diamond tools, but would add thermal distortion and residual stress in the cut material to the problem. The laser cutting method studied in this section avoids this melting process, however. A basic idea for the method is schematically shown in Fig. 16.28. The local thermal stress is generated ahead of the crack tip by laser irradiation without melting the material, which is a driving force for crack propagation. The method is a possible solution to the aforementioned brittle material cutting issues, which have been studied analytically as well as experimentally in previous works. Practical application of this new laser method is still limited, however, due to the difficulty of controlling crack propagation behavior. In this section, the applicability of twin-beam irradiation for laser cutting as a method of controlling crack propagation behavior is presented based on the results of FEM analysis.

A soda-lime glass with a thickness of 1.77 mm was used as an example of a brittle material. A starter notch was introduced using a diamond cutter at the starting point of laser irradiation. The laser equipment used was a Nd:YAG laser. The laser power and scanning speed were fixed at 500 W with continuous wave (CW) and 20 mm/s, respectively. The focusing point was adjusted to 3 mm up from the specimen surface to avoid the melting of the material. The laser beam began irradiating from the edge of specimen and continued to scan straight up to 10 mm and then changed the scanning direction of the laser beam to angles, ψ = 30, 60 and 90 degrees.

Figure 16.29 shows an example of a laser cutting line for ψ = 90°. The figure shows that the laser cutting line was swerved from the original laser scanning line. The crack propagation direction did not follow the laser scanning direction from 1.4 mm before the turning point of laser scanning direction. Similar results were obtained at other angles too. It is considered that the crack follows 1.4 mm behind the laser spot and the crack propagation direction is changed immediately when the laser scanning direction is changed.

The crack propagation behavior was estimated using FEM analysis under the same condition of laser irradiation as the experiment shown above. In the crack path estimation, a criterion for crack propagation is assumed as K_θmax > K_c, where K_θmax is a stress intensity factor for the maximum tangential stress generated by the laser irradiation, and K_c is the fracture toughness of the material. The crack propagation direction is also assumed to be the direction of K_θmax. The crack propagation path obtained using FEM analysis is also shown in Fig. 16.29. The crack propagation behavior estimated by the FEM analysis corresponds well with the experimental results. A stress intensity factor for the maximum tangential stress, K_θmax, is therefore considered to be a dominant parameter for crack propagation. Crack growth direction can be controlled by controlling the maximum tangential stress, i.e. controlling the thermal stress distribution induced by the laser irradiation.

In order to control the crack growth direction during the laser cutting, a twin-beam laser technique was studied using FEM analysis. Laser cutting using the tandem twin beam is schematically shown in Fig. 16.30. The two beams are placed parallel to the laser scanning direction, which is called the tandem twin-beam configuration as shown in the figure, and the position of the crack tip is in between the front and the back beams. The back beam moves toward ϕ, which is equal to the expected angle for the change of crack path, when the crack tip reaches the expected position for changing the crack propagation direction. The expected angle for the change of crack path was 30° in this analysis. The distance between the front and the back beams was 1 mm with a laser spot radius of 0.35 mm. Figure 16.31 shows the change in stress intensity factors for Mode I and Mode II, i.e. K_I and K_II, due to the change in the intensity ratio between the front and the back beams. The mode ratio K_I/K_II is changed depending on the beam intensity ratio. This means that the direction of the maximum tangential stress can be controlled by the twin-beam irradiation. Figure 16.32 shows an example of crack growth behavior estimated by FEM analysis for the tandem-beam irradiation with intensity ratio of 1:2 (front:back) at an expected angle for crack growth of 30°. As can be seen from the figure, the crack propagation direction followed the laser scanning direction. The twin-beam method could therefore control the crack propagation direction by changing the beam intensity ratio. Parallel twin-beam laser cutting, where two beams lined perpendicular to the laser scanning direction, was also studied [14], but the result indicated that it was not an effective method of controlling the crack path.

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